The Sierras de Tandil is NW-SE trending mountain system, 300 km long with altitudes between 50 and 250 mts above the surrounding pampean plain of Buenos Aires province. The range is composed of Proterozoic crystalline basement which belongs to the Brazilian craton, unconformably overlain by Precambrian and lower Paleozoic sedimentary rocks. The purpose of this contribution is to analyse the geomorphological evolution of the range within the frame of the tectonic history of the South Atlantic Ocean and the sedimentary filling of neighboring basins.

Three planation surfaces are recognized along the mountain range and interpreted as long-term geomorphic features. The oldest planation surface (pre-Cretaceous or prerift surface) is represented by the levelled summits of the highest ranges near Tandil (altitudes above 450-500 asl). No weathering profiles are associated with this surface.

The intermediate planation surface (late Jurassic-early Cretaceous to mid-late Tertiary or postrift surface) is the most extensive with altitudes increasing from SE to NW (200 mts to 350 mts) and dipping slightly towards the SW. Corestones, concentric weathering rinds, granitic gruss and kaolinized crystalline rock profiles are associated with this surface. In some areas, present soils are partly developed on weathered granitoid rocks. The characteristics and distribution of the weathering features suggests they correspond to the root of a weathering mantle which was probably stripped away in the early to mid Cenozoic. The youngest planation surface (initiated during the Oligocene?-early Miocene?), displays some moderate weathering and is covered by a late Cenozoic apron (Pliocene-Pleistocene and Holocene) of eolian deposits (mostly loess and loess-like deposits) over which present soils are mostly developed.